This invention relates to anti-backlash nuts in general and, more specifically, to an adjustable pre-load nut of the anti-backlash type disclosed by us in U.S. Pat. No. 4,249,426 which was reissued as RE 32,433 on Jun. 9, 1987.
Our above-identified patent discloses an anti-backlash nut which is designed to translate back and forth along a screw in opposite directions. The screw has external threads and is driven by a reversible motor. The nut, per se, comprises a nut body portion having internal threads complementary to the external threads on the screw. The body portion has means for attaching it to a load which is supported and reciprocated by the nut. The nut includes a plurality of longitudinal flexure members cantilevered to the body portion at one end and terminating at a free floating end. The flexure members extended parallel to one another and to the axis of rotation of the screw and include internal threads complementary to the threads of the screw. The screw threads are helical as are the threads of the nut.
On the free floating end of each flexure member is a ramp extending radially outwardly in a direction away from the fixed or cantilevered end of the flexure members. The ramps are surrounded by an annular pressure applying ring which is continuously forced away from the nut body upwardly along the ramps by a compression spring. The force against the ramps cause inwardly extending force vectors to constantly press the threads of the flexure members against the threads of the screw as the nut translates back and forth lengthwise along the screw.
This construction results in a substantially constant load and constant torque sufficient to drive the nut and the load that the nut normally carries under normal working conditions.
There are variations in the torque required to drive the nut along the screw and frequently adjustments to that torque are desirable. These variations can result from carriage drag, indeterminate bearing efficiencies and drag caused by the rails on which the nut translates, etc. These variations can be compensated by changing the size and strength of the spring. This necessitates removing the nut from the lead screw and replacing the spring, all of which is time-consuming and inefficient.
It is one object of the present invention to be able to vary the torque application without removing the nut or the spring.
Another object of this invention is to vary the torque by merely adjusting the nut.
The force with which the external threads of the flexible members engage the threads of the screw is maintained constant even though the threads wear over a period of time. This counteracts backlash and is because the pressure applying ring is at all times urged by the spring toward the large end of the ramps and will literally move in that direction at a very small rate over a lengthy period of time of operation.
Yet another object of this invention to be able to vary the torque either independently of backlash compensation or in combination with backlash compensation. These objects and other features of the invention will become apparent with reference to the accompanying specification and drawings.